1. Field of the Invention
The present invention relates to a lubricant for refrigerating machines employing a refrigerant comprising hydrofluorocarbon such as tetrafluoroethane. More particularly, it relates to a lubricant for refrigerating machines which is used along with a refrigerant comprising a substituted fluorohydrocarbon (hereinafter sometimes referred to as "flon compound") such as 1,1,1,2-tetrafluoroethane (R-134a). As such, the lubricant is excellent in the performance such as wear resistance, electrical insulating properties, hydrolytic stability, nonhygroscopicity and the like and further has excellent returnability of the lubricant in a compression-type refrigerating cycle (the characteristics that the lubricant is inclined to return to a compressor after it is circulated in the refrigerating cycle).
2. Description of the Related Arts
Conventionally, in a compression-type refrigerating cycle comprising a compressor, a condenser, an expansion valve and an evaporator, a flon compound including a fluorinated hydrocarbon such as dichlorodifluoromethane (R-12) and chlorodifluoromethane (R-22) has been used as the refrigerant. Many kinds of lubricants compatible with them have also been produced industrially available for use.
However, there have been misgivings that these flon compounds widely used as the refrigerant are liable to cause depletion of the ozone layer and environmental pollution after they are released into the open air. In an effort to prevent environmental pollution, fluorohydrocarbons (or chlorofluorohydrocarbons), for example 1,1,1,2-tetrafluoroethane (R-134a) and the like have been developed as a possible replacement in recent years. Until now there have been introduced into market many so-called substituted flon compounds with minimized danger of environmental pollution and capable of satisfying said required properties, for example 1,1,2,2-tetrafluoroethane (R-134) and the like, as well as R-134a.
These new substituted flon refrigerants are different from conventional flon refrigerants in characteristics. Thus, glycol compounds, ester compounds and the like have been proposed as the lubricant for use in combination with them (U.S. Pat. No. 4,755,316 and Japanese Patent Application Laid-Open No.33193/1991), and these patents are intended to dissolve substituted flon refrigerants and lubricants completely so as to provide good returnability of the lubricants.
In an attempt to circulate all over a refrigerating cycle a refrigerant and a lubricant in their completely dissolved state as envisaged by these patents, however, said lubricant must be selected from only a very limited number of compounds acceptable to the chemical properties of substituted flon refrigerants such as R-134a. Accordingly, the selected lubricant is forced to sacrifice certain aspects of the important performance required as the lubricant.
In fact, polyalkylene glycol compounds and ester compounds thereof, both known as compatible with R-134a, have been found to have insufficient wear resistance, along with faulty electrical insulating properties. It has also been found that the polyalkylene glycol compounds are highly hygroscopic and the ester compounds thereof are hydrolytically unstable. Thus the demand for a solution in these problems has been raised in relevant industrial segments.
The present invention has been completed on the basis of a concept quite contrary to ordinary common knowledge as set forth above, in an effort to find a solution in the problems. As the result, it has been found that all these aspects of the performance can be unified by using a substance excellent in wear resistance, insulation resistance and the like as the lubricant, compounding therewith a fluidity improver to improve the fluidity of the lubricant in the presence of a refrigerant (R-134a and the like) and thus providing good returnability of the lubricant in a compression-type refrigerating cycle. It has thus been made possible that a lubricant combined even with a new refrigerant has all the required performance including excellent wear resistance, electrical insulating properties, hydrolytic stability and the like and further ensuring excellent returnability of the lubricant. The present invention has been accomplished based on this finding.